Government scientists at the centre of a botched study in which cow brains were used in a test for BSE in sheep were most likely confused by badly labelled sample bottles, an independent auditor said on Friday.
Officials said last month that Britain's largest animal research laboratory had tested samples thought to be from sheep for a study looking for evidence of mad cow disease in the national flock but later found the material was from cows.
Two investigations into the mix-up said the Institute for Animal Health had failed to label and trace samples accurately, with one auditor saying it had committed a classroom error and forgotten to mark which species was contained in each bottle.
"Labelling didn't give the species," Helen Wilkinson, from independent consultants Risk Solutions, told a news conference.
"The bags were labelled but it wasn't labelled on the bottles of samples."
The director of a group sponsoring the institute said bags containing hundreds of bottled samples of brain matter from cows and sheep had not been labelled clearly and had been moved from their separate freezers to just one.
Professor Ray Baker from BBSRC said the turnover of laboratory technicians might have contributed to the schoolboy error, which has further embarassed a government under fire over its handling of a prolonged foot-and-mouth epidemic.
But he denied the affair had dented the reputation of the organisation and also questioned whether the institute should bear full responsibility for the mistake.
"The Institute for Animal Health works at the highest level of science. It has an international reputation which is undoubted," he said.
"We are not yet convinced entirely that the mistake occurred in the IAH," he said, adding that the blunder did cast doubt about good laboratory practice - something he learned in the classroom as a 15-year-old.
The IAH has said a separate government laboratory had made errors in managing samples.
Another auditor, the United Kingdom Accreditation Service (UKAS), said it found "weak links" at the institute, which some scientists have said could pose questions about the rigour of other research into BSE, or bovine spongiform encephalopathy.
"We found the places where mistakes could have been made," Bryan Smith from UKAS said.
Government officials played down the gaffe, saying the IAH study was one several projects and that the experiment was in doubt because it was based on brain taken from animals a decade ago and for another experiment.
Elliot Morley, animal health minister, said there was no evidence to suggest sheep could contract BSE and there was no reason to avoid eating lamb.
Officials say there is a theoretical risk that BSE could be masked by scrapie - a similar type of disease in sheep believed to pose no threat to human health.
BSE's human equivalent, vCJD, has claimed over 100 lives.
The UK independent Consumers Association called for more proof that there was no risk from lamb.
"While we note that DEFRA is carrying out further research, we believe that consumers have a right to know when the government will have an answer to the question - is BSE in or has it ever been in sheep?" said spokeswoman Mona Patel.
"Consumer confidence in science and the government's ability to ensure the safety of our food will have been dented by this fiasco, and it is important that the government is able to answer this fundamental question."